The properties of pure dimethylpolysiloxanes are favorably influenced in a number of ways by the introduction of the diphenylsiloxy group. The introduction of the diphenylsiloxy group is known to improve the radiation resistance, thermal resistance, noncombustibility, low-temperature flexibility, and so forth. Diphenylsiloxane-dimethylsiloxane copolymers have heretofore been prepared by the alkali metal-catalyzed ring-opening equilibration copolymerization of cyclic diphenylsiloxane and cyclic dimethylsiloxane.
One drawback to the application of this prior art has been that it gives large amounts of cyclic by-products at equilibrium, which reduces the yield of the copolymer product. This also makes control of the molecular weight problematic. Another drawback to the prior art occurs when water or an alpha,omega-dihydroxyoligosiloxane is used as endblocker for the purpose of producing copolymer terminated by silanol at both molecular chain ends. These silanol groups undergo dehydration condensation under the conditions used for copolymerization, which results in the production of only copolymer with a molecular weight much higher than expected.
The present inventor has already proposed a method for the preparation of diphenylsiloxane-dimethylsiloxane copolymers in U.S. Pat. No. 5,169,920. This method, which uses the lithium metal salt of organosilane or organopolysiloxane as polymerization initiator, discloses the nonequilibration polymerization of a mixture of hexaphenylcyclotrisiloxane and hexamethylcyclotrisiloxane followed by termination of the reaction. This polymerization is carried out in solvent at temperatures up to the boiling point of the solvent. Since this preparative method generates only small amounts of cyclic by-product, its copolymer yields are higher than in the conventional methods and it makes possible control of the molecular weight based on the total amount of polymerization initiator and molecular-weight regulator used. Moreover, since this polymerization is run at temperatures sufficiently low to substantially retard silanol group condensation, this method makes possible production of a silanol-endblocked copolymer while exercising control over the molecular weight. Nevertheless, this preparative method still does not provide complete inhibition of equilibration reactions and silanol condensation. As a result, it yields a copolymer product with a molecular weight somewhat higher than the calculated value and a broadened molecular weight distribution with the appearance of a shoulder on the high molecular weight side in gel permeation chromatography (GPC).